Tag: hygiene hypothesis

Bacteria are practically everywhere around us, including on and inside you, but that is in many ways a good thing. For instance, having a diverse set of microbes living on your skin might help prevent allergies. A new study published in PNAS links two factors related to how microbes might affect our health: the observation that diversity of microbes on a person is related to the diversity of microbes in their environment, and the hygiene hypothesis, which suggests that the modern uptick in allergies and autoimmune diseases is caused by childhood under-exposure to bacteria.

For a while now, scientists have known that kids living on farms are less likely to have allergies or asthma. Being around livestock means the farm kids are also around a more diverse set of bacteria than city kids living in an apartment. In this new study, scientists swabbed the skin bacteria of 118 Finnish kids, some who lived in rural areas and some who lived in urban areas. They also tested the kids for levels of an antibody called IgE, high levels of which indicate hypersensitivity to allergens, or what is known as atopy. Lastly, they surveyed the parents about plant diversity around their homes.

With modern plumbing and hygiene, the number of nasty microbes we humans are exposed to has plummeted, while the rate of autoimmune diseases and allergies has shot up. Are those related? Proponents of the hygienehypothesis think so: our immune system is supposed to develop by encountering microbes, so being too clean throws it out of whack as the immune system overreacts to minor insults.

A new study found that mice raised germ-free had especially high numbers of invariant natural killer T cells (iNKT) in their colons and lungs—the mouse versions of inflammatory bowel disease and asthma, respectively. Most evidence supporting the hygiene hypothesis has just been in observed correlations, so this research that identifies a plausible molecular mechanism is good evidence for how over-cleanliness might cause immune dysfunction.

Cleanliness is a virtue, but it’s possible to overdo it–that’s the message from a new study, which found that antibacterial soap may be doing teenagers more harm than good. The study found that the more teenagers are exposed to the antibiotic triclosan, the more likely they are to suffer from allergies and hayfever.

The researchers also looked at the effects of the widely used plastic chemical Bisphenol A (BPA), and found signs that teenagers with more BPA exposure may have immune system problems. The study was the first of its kind to examine the link between these two chemicals and immune dysfunction, which had only previously been studied in animals. Both chemicals are endocrine-disruptors, which means they may mimic or interfere with the body’s natural hormones.

“Many research studies show an association between exposure to environmental chemicals and different disease outcomes. There is a lack of data, however, examining whether exposure to these chemicals may affect our immune systems,” Erin Rees Clayton, a researcher from the University of Michigan school of public health said in an email. [The Montreal Gazette]

Biologist Dorothy Matthews and company wanted to test a particular bacteria, Mycobacterium vaccae. It’s found commonly in the soil and carried widely through the air, so if you take a walk in the park you’ll probably breathe it in. Previous studies have shown that the bacterium increases serotonin in the brain, and have even suggested that the bacterium has antidepressant qualities. Since the neurotransmitter serotonin is also involved in cognition, the team wanted to see if the bacterium could have a direct effect on learning. Indeed it did, Matthews’ team announced at the General Meeting of the American Society for Microbiology in San Diego.

In a classic test of learning ability, Matthews gave mice a treat – white bread with peanut butter – as a reward to encourage them to learn to run through a maze. When she laced the treat with a tiny bit of Mycobacterium vaccae, she found that the mice ran through the maze twice as fast as mice that were given plain peanut butter [New Scientist].

This week brings more vindication for a childhood full of bumps, bruises, and going outside, rather than sterile modern living. In a long-term study published in The Proceedings of the Royal Society B, U.S. researchers suggest that over-cleanliness could make babies more prone to inflammation later in life, and in turn raise the risk for stroke and heart disease.

Thomas McDade’s team studied more than 1,500 people in the Philippines who had health surveys at age two and then again at age 20. The team tested them for C-reactive protein (CRP), a marker of inflammation. They found that the more pathogens the people had encountered before age 2, the less CRP they had at age 20. Every episode of diarrhoea back then cut the chance of higher CRP later by 11 per cent; every two months spent in a place with animal faeces cut it by 13 per cent. Being born in the dusty, dirty dry season cut the chance by a third [New Scientist].

The fear surrounding nut allergies among children has gotten so out of control, one doctor says, that it could be considered an outbreak of mass psychogenic illness (MPI), more informally known as mass hysteria. Writing in the British Medical Journal, medical sociologist Nicholas Christakis argues that a tiny fraction of hospital admissions and deaths are due to allergic reactions to nuts yet ever more draconian measures are being brought in to prevent any child coming into contact with nuts [Telegraph]. Those draconian measures fuel parents’ anxieties, Christakis says, in what he calls a “cycle of over-reaction.”

Christakis cites the extreme example of when a potentially fatal peanut was “spotted on the floor of a school bus, whereupon the bus was evacuated and cleaned (I am tempted to say decontaminated), even though it was full of 10-year-olds who, unlike two-year-olds, could actually be told not to eat food off the floor” [The Register]. He also mentions some school policies of banning all nuts, peanut butter, and even baked goods that may have come into contact with nuts. While he acknowledges that nut allergies can be serious and even deadly, Christakis says that reasonable preventative measures could protect vulnerable children without scaring the bejesus out of everyone else.

A microbe that has caused trouble in human stomachs for around 60,000 years may also play a role in preventing children from developing asthma and other allergies. In a new study, researchers say that a current campaign to wipe out the bacterium, Helicobacter pylori, may be having the unintended consequence of boosting asthma rates in kids.

A longtime resident of the human stomach, H. pylori went largely undetected until Australian scientists discovered it in 1979 and went on to show that it can cause stomach ulcers. Further work has linked it to stomach cancer. It’s now treated with antibiotics whenever detected [Science News]. But researchers say that when they studied health records of over 7,000 kids between the ages of 3 and 13, they found that children with H. pylori in their stomachs were less than half as likely to develop asthma. Those children were also less likely to suffer from eczema and hay fever.